1. Field of the Invention
This invention relates to a special effect apparatus and, more particularly, to a special effect apparatus for improving the picture quality of edges when performing a picture converting operation.
2. Description of Related Art
Recently, pictures prepared by exploiting special effect artifices, such as by wrapping a signal-processed video image around a curved surface and synthesizing the image with the background, or by, as it were, affixing an image on a paper sheet and bending the image in the manner of folding a page of a book, have come to be used in increasing numbers in broadcast TV programs.
When the background image is simply synthesized with the signal-processed image (image corresponding to the foreground) to form an image, the synthesized image is displayed by switching in accordance with key signals inputting the two images. These key signals are generated in accordance with input video signals. Therefore, if a boundary line or an interface between the two images abruptly changes the input key signals are also abruptly changed, that is, switched acutely. Since high frequency components are contained in the key signals causing the acute switching, the interface between the two images synthesized in accordance with the key signals is necessarily disturbed.
When a video signal as shown in FIG. 5(A) is entered, a key signal is formed, which key signal has a rising edge and a falling edge temporally surrounding the rise and decay timing of the input video signal, as shown in FIG. 5(B), in order to prevent the key signal level from being changed acutely at the rise and decay timings of the input video signal for smoothing the disturbances in the edge boundary region brought about by simple signal synthesis. By raising the gain of the key signal to clip its lower level, an adjusted key signal shown in FIG. 5(C) is produced. By multiplying the key signal shown in FIG. 5(C) by the input video signal shown in FIG. 5(A), a synthesized image having a smooth edge boundary between the component images, as shown in FIG. 5(D), is produced.
However, in the synthesized output video signal, shown in FIG. 5(D), a visual nipping or intruding effect is produced in the boundary line between the surrounding background image and the signal-processed image.
It is noted that a mean value between the signal level of an area corresponding to the surrounding background image and the signal level of an area of the signal-processed image represents a boundary line of the synthesized image. Since the synthesized image is obtained as a function of the key signal adjusted as shown in FIG. 5(C), the level of the mean value between the maximum and minimum values of the adjusted key signal represents the boundary line of the component images of the synthesized image. Since the boundary line displayed at the input video signal start position is synthesized by the adjusted key signal, the edge is seen to be pushed or intruded towards the inside of the video signal curve by a level difference between the level of the input video signal start position and the level of the boundary position of the adjusted key signal, that is a level difference d shown in FIG. 5(D), resulting in a correspondingly reduced size of an image combined from the foreground and the background.
In this manner, for effecting a smooth display, the boundary line between the background and the video signal synthesized thereto has a small width, when viewed as a picture, and looks as if the center of the width of the boundary line were shrinking towards the inside region of the video signal.
On the other hand, if the background and the component digital signals are combined using the luminance signals and chrominance signals having a different bandwidth from the luminance signals as input video signals so as to be based on the Db 1 format used in a so-called 4:2:2 component digital video tape recorder, there are produced, by the above described conventional special effect apparatus, portions in which only luminance signals exist, that is, portions devoid of color.
TABLE 1 ______________________________________ sample number 0 1 2 3 4 5 6 E O E O E O E ______________________________________ luminance * * * * * * * signal (Y) Chrominance * * * * signal (C.sub.R) Chrominance * * * * signal (C.sub.B) ______________________________________ E: evennumbered sample O: oddnumbered sample *: indicated that the signal is processed by sampling
The above described phenomenon is brought about by the manner of sampling according to the above mentioned D1 format. Referring to Table 1, the luminance signal Y as one of the component signals is sampled at a frequency of 13.5 MHz, whilst the chrominance signals (C.sub.R and C.sub.B) as the other component signals are sampled at a frequency of 6.75 MHz which is half the sampling frequency of the luminance signals, for example, at even numbers (0, 2, 4, 6, . . . ) of the sampling numbers (0, 1, 2, 3, . . . ) of the luminance signals.
If the luminance signal (Y) and the chrominance signals (C.sub.R, C.sub.B) are sampled at the timings of the same even sampling numbers, there is no portion devoid of color in the synthesized picture. However, in case of odd numbered sampling with a shift of one clock period with respect to the even numbered sampling, chrominance signal sampling is not made, so that only the luminance signal is displayed for the odd-numbered sampling. That is, portions devoid of color are produced in the image. Furthermore, due to sampling deviation of the chrominance signals, chrominance signals are displayed in those portions which are devoid of luminance signals.